Hi Alex,

On Sat, Feb 28, 2026 at 08:34:37PM -0800, Alexander Coffin wrote:

From personally building the kernel a few months ago (reproducibly) one of
the parts that was incorrect in the docs was that is says to use KCFLAGS
when you should use KCPPFLAGS for the following. I had forgotten to report
this until now and I'm not sure exactly what is expected to make a patch to
the Kernel docs, so I am hoping someone else can do the probably one line
fix (or maybe 2 if you count the other reference to KCFLAGS in the file).

quoted

When the kernel is built out-of-tree, debug information may include
absolute filenames for the source files.  This must be overridden by
including the ``-fdebug-prefix-map`` option in the `KCFLAGS`_ variable.

from `Documentation/kbuild/reproducible-builds.rst`.

Thanks for bringing this up. I assume this is due to the debug
information generated in assembler files. While the KCPPFLAGS suggestion
works because it is included for both .c and .S compilation, I think it
would be more proper to recommend applying '-fdebug-prefix-map' to
KAFLAGS in addition to KCFLAGS to insure the flag is only added when
truly needed, just as a defensive measure against compiler and Kbuild
changes. I can send a patch for this later.

Cheers,
Nathan